Baghdad: The last US combat brigade pulled out of Iraq at dawn on Thursday, a key milestone in the withdrawal of American forces more than seven years after the US-led invasion ousted dictator Saddam Hussein.

Under the cover of dark, the 4th Stryker Brigade, 2nd Infantry Division, crossed the border into neighbouring Kuwait ahead of the planned declaration of an end to US combat operations in Iraq by an August 31 deadline.

The pullout came two days after a suicide bomber killed 59 people at a Baghdad army recruitment centre in Iraq’s deadliest attack this year, sparking concern the country’s forces are incapable of handling security on their own.

“Yes, they did," Lieutenant Colonel Eric Bloom said, when asked if the 4th Stryker Brigade had crossed into Kuwait. “The last one crossed at about 6:00 am this morning."

“They have a few more days to clean the equipment, prepare the equipment, get it ready for shipment, and then they’ll fly out (back to the United States)."

It took two days for the 360 military vehicles and 1,200 soldiers to travel from Camp Liberty on Baghdad’s outskirts and Camp Taji north of the capital, through the Shiite south, and into the Gulf emirate, Bloom said.

He said the remaining solders in the 4,000-strong brigade departed the country by air.

Captain Russell Varnado at Camp Arifjan, a major US base about 70 kilometres (45 miles) south of Kuwait City, told AFP that “the combat troops have finished moving."

“The troops are transitioning now. They are scheduled to go back home soon," he said, without giving a specific date.

Kuwait, which is host to several American military camps in the country’s northern desert close to the Iraqi border as well as a naval base, was used as the launchpad for the US-led invasion.

About 56,000 US soldiers remain stationed in Iraq, with that figure set to drop to 50,000 by September 1, less than a third of the peak level during “the surge" of 2007.

At that point, the US mission in Iraq will be re-christened “Operation New Dawn", from “Operation Iraqi Freedom" — the name given to American operations here since the 2003 invasion.

The remaining 6,000 soldiers who will leave the country in the next two weeks are clustered throughout Iraq, Captain Sarah Baumgardner said.

“And we’ll continue to go through our responsible drawdown to meet that drawdown by 1 September," major general Stephen Lanza said in an interview with US television channel MSNBC. “It is about a transition to a change of mission, going from combat operations to stability operations."

The pullout coincided with the arrival of James Jeffrey, the new US ambassador to Iraq, who presented his diplomatic credentials Wednesday to the conflict-torn nation’s head of state, President Jalal Talabani.

Jeffrey’s arrival comes during a political deadlock in Iraq, with no new government having yet formed after elections in March, and in the middle of the holy Muslim fasting month of Ramadan, when insurgent attacks typically peak.

A spike in violence in July, the deadliest month here since 2008 according to Iraqi figures, and the suicide attack on Tuesday which was blamed on Al-Qaeda, have sparked disquiet over the readiness of Iraq’s security forces.

While US officers insist their Iraqi counterparts are up to the task, the country’s top military officer told AFP last week that US forces may be needed in the conflict-wracked nation for a further decade.

That seems unlikely, however, as the White House has repeatedly insisted that the withdrawal schedule, which will see the last US soldier depart Iraq at the end of next year, remains on track.

US state department spokesman Philip Crowley described the end of combat operations as a “historic moment", but stressed America’s long-term commitment to Iraq was unwavering.

“We’re not ending our involvement in Iraq," he told MSNBC as the footage of the brigade crossing into Kuwait was shown.

“We will have important work to do. This is a transition. This is not the end of something. It’s a transition to something different. We have a long-term commitment to Iraq."

Crowley said the conflict had come “at high expense" after one trillion dollars was spent in Iraq and more than 4,400 lives were lost.

In a letter dated August 18 and posted on the White House website, President Barack Obama also hailed the end of combat operations but made no mention of the final combat troops leaving.

“Shortly after taking office, I put forward a plan to end the war in Iraq responsibly," the letter said.

“Today, I’m pleased to report that — thanks to the extraordinary service of our troops and civilians in Iraq — our combat mission will end this month, and we will complete a substantial drawdown of our troops."